Brighthand have an editorial about the recent Origami Project saying '
Ultra Mobile PCs are going to be very important'.
Ed Hardy (Brighthand's Editor) feels that the new UMPCs will be bought by people who used to buy high end PDAs or those looking for a lighter laptop replacement.
Personally, I find my M5000 PDA/phone a bit too bulky so I'm not really interested in a UMPC as a high end PDA replacement. It's a bit more appealing as a laptop replacement but I've been having problems with my present laptop running a bit slow when it has a few apps open and it's a lot better spec'd than the UMPC.
There probably is a niche for these things - probably running EMRs or in similar one-app environments where the processing power of a laptop isn't required. In fact UMPCs might be ideal for people who are considering a tablet but have held out for price reasons.